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UK police reopen Madeleine McCann case

Madeleine when she was four (left) and a computer-generated image of her at nine (right). (AAP)

British police are opening their own investigation into the 2007 disappearance of toddler Madeleine McCann in Portugal.

They have identified 38 people of interest across Europe.

A two-year review of the original Portuguese investigation, which is officially closed, had resulted in some new evidence, said Detective Chief Inspector Andy Redwood of Scotland Yard, the officer in charge of the probe.

Police also still believe that Madeleine, who was three when she disappeared, may be alive, he said.

"We have identified 38 persons of interest from a number of European countries. Twelve of those people are UK nationals who we believe were in Portugal at the time Madeleine disappeared," Redwood said.

Neither Madeleine's parents Gerry and Kate, nor the friends who were having dinner with the McCanns when she disappeared, were among the suspects.

The McCanns say it is a "huge step forward" in their ongoing search for their daughter.

The couple launched a global campaign to find Madeleine after she vanished from the family's holiday flat in Praia da Luz on Portugal's south coast in May 2007, while they were eating dinner nearby.

Pictures of the smiling blonde girl was plastered on newspapers, posters and websites across the world, although numerous possible sightings came to nothing.

Scotland Yard say they believe she is still alive.

Since the ongoing review was launched in 2011, British police have gathered more than 30,000 documents, visited Portugal 16 times, carried out new witness interviews and generated new theories.

"The review has given us new thinking, new theories, new evidence and new witnesses," Redwood said.

"That has given us the ability to see this case with fresh eyes."

He added: "I believe critically that this is an important moment for Madeleine."